In the production of optical glass fibers, one or more coating layers are usually applied to the optical glass fibers immediately after drawing to protect the glass surface from detrimental moisture and/or mechanical attack which would otherwise occur. These coating layers have been formed from UV-curable coating compositions consisting of organic UV-curable oligomers, reactive diluents, thermal intitiators, photo-initiators, stabilizers, and coupling agents. These UV-curable coating compositions should be solvent-free.
UV-curable coating compositions used in telecommunications sometimes include a UV curable urethane acrylate oligomer containing a vinyl copolymer. According to EP-A-204497, urethane acrylates having a vinyl addition polymer as backbone are not well suitable for use in optical fiber coating compositions. A further disadvantage in the use of UV-curable urethane acrylate oligomers containing a vinyl copolymer is caused by the fact that, in current practice, the vinyl copolymer is first synthesized in a solvent and then a functional group present on the vinyl copolymer is subjected to a further reaction so as to provide an acrylate functional group on the copolymer. The solvent(s) is then removed from the acryrated vinyl copolymer to provide a substantially solvent-free composition.
Such a process is disclosed in U.S. Pat. No. 4,537,667. In this patent, a UV-curable copolymer is synthesized in a solvent that is later removed. The solvent can be inorganic or organic. In Example 1 of this patent, both an inorganic solvent and an organic solvent were used to synthesize the UV-curable copolymer and then they were removed. The solvent is commonly a relatively volatile solvent and the removal thereof frequently requires use of a vacuum.
However, it is very difficult to remove the last traces of the solvent under conditions which will not adversely affect the nature of the urethane acrylate copolymer. Thus, almost invariably traces of solvent will remain in the UV-curable coating composition, which may undesirably affect the properties of the cured coating.
Furthermore, the removal of the solvent is an additional step which requires the use of additional equipment, energy consumption, and is environmentally unfriendly. Many solvents cause undesirable effects on humans and the environment, and thus special equipment is required to prevent contamination of the environment or exposure to humans. All of these aspects add considerably to the cost of the composition.
Other teachings in the prior art have referred to the use of reaction solvents which are free from active hydrogen atoms such as are on hydroxyl groups; see U.S. Pat. Nos. 3,694,415 and 3,719,638, and U.K. Specification 1,281,898. U.S. Pat. No. 5,171,760, described the undesirable aspects of solvent-based reaction systems; however, it uses a different technological approach from that of the present invention. Japanese published application 5 701 2021 describes a still different approach of using an unsaturated urethane oligomer for reaction with an hydroxy-group containing vinyl polymer.
There is and has been a need for an effective method of preparing a solvent-free, radiation-curable coating composition which does not require the use of a solvent that must be removed.